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          const in C++
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        <p>C++中const可用于变量, 指针, 函数和类.</p>
<h2 id="修饰变量"><a href="#修饰变量" class="headerlink" title="修饰变量"></a>修饰变量</h2><p>const修饰的变量的值不能再修改, 编译器也可以对其进行数据静态类型检查. 所以用const声明的变量可以保证类型安全, 用以取代C中的宏定义.</p>
<p>const定义的常量在C中是全局的, 在C++中则视声明位置而定.</p>
<h2 id="修饰指针"><a href="#修饰指针" class="headerlink" title="修饰指针"></a>修饰指针</h2><p>使用指针时涉及到两个对象: 该指针本身和被它所指的对象.</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> <span class="keyword">char</span> *pc;    <span class="comment">// pc是到const char的指针, 即*pc的内容不可修改, 但pc可以指向其他字符串</span></span><br></pre></td></tr></table></figure>

<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">char</span> <span class="keyword">const</span> *pc;    <span class="comment">// 同"const char *pc"</span></span><br></pre></td></tr></table></figure>

<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">char</span> *<span class="keyword">const</span> pc;    <span class="comment">// pc到char的const指针, 即*pc的内容可以修改, 但pc不能指向其他字符串</span></span><br></pre></td></tr></table></figure>

<a id="more"></a>

<h2 id="函数中使用"><a href="#函数中使用" class="headerlink" title="函数中使用"></a>函数中使用</h2><h3 id="修饰函数参数"><a href="#修饰函数参数" class="headerlink" title="修饰函数参数"></a>修饰函数参数</h3><p>const常用于修饰函数的指针参数和引用参数, 一是表示参数的内容不可修改, 二是可以提高效率.</p>
<h3 id="修饰函数返回值"><a href="#修饰函数返回值" class="headerlink" title="修饰函数返回值"></a>修饰函数返回值</h3><p>如果函数的返回值是指针或引用, 增加const修饰后, 则返回值不能被直接修改, 且返回值只能被赋值给const修饰的同类型变量.</p>
<h2 id="类中使用"><a href="#类中使用" class="headerlink" title="类中使用"></a>类中使用</h2><h3 id="修饰成员变量"><a href="#修饰成员变量" class="headerlink" title="修饰成员变量"></a>修饰成员变量</h3><p>const的成员变量不能被修改, 且只能在初始化列表中赋值.</p>
<h3 id="修饰成员函数"><a href="#修饰成员函数" class="headerlink" title="修饰成员函数"></a>修饰成员函数</h3><p>任何不会修改数据成员的成员函数都应用const修饰.<br>修饰形式:</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">func</span><span class="params">(<span class="keyword">int</span>)</span> <span class="keyword">const</span></span>;</span><br></pre></td></tr></table></figure>

<p>对于使用了const修饰的成员函数:</p>
<ul>
<li>不能修改成员变量</li>
<li>不能调用非const成员函数</li>
<li>const类对象只能调用const成员函数</li>
<li>const不能与static同时使用. const其实是用于修饰this指针, 即this指向的内容不可修改. 但static修饰的静态成员是类的而不是类对象的, 不含有this指针. 所以const和static不能共用.</li>
</ul>
<p>另外, 构造函数和析构函数也是成员函数, 但不能使用const修饰, 因为两者都是在对成员变量修改.</p>
<h3 id="修饰类对象"><a href="#修饰类对象" class="headerlink" title="修饰类对象"></a>修饰类对象</h3><ul>
<li>对象的任何成员不能被修改</li>
<li>只能调用const成员函数</li>
</ul>

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          <div class="post-toc motion-element"><ol class="nav"><li class="nav-item nav-level-2"><a class="nav-link" href="#修饰变量"><span class="nav-number">1.</span> <span class="nav-text">修饰变量</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#修饰指针"><span class="nav-number">2.</span> <span class="nav-text">修饰指针</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#函数中使用"><span class="nav-number">3.</span> <span class="nav-text">函数中使用</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#修饰函数参数"><span class="nav-number">3.1.</span> <span class="nav-text">修饰函数参数</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#修饰函数返回值"><span class="nav-number">3.2.</span> <span class="nav-text">修饰函数返回值</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#类中使用"><span class="nav-number">4.</span> <span class="nav-text">类中使用</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#修饰成员变量"><span class="nav-number">4.1.</span> <span class="nav-text">修饰成员变量</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#修饰成员函数"><span class="nav-number">4.2.</span> <span class="nav-text">修饰成员函数</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#修饰类对象"><span class="nav-number">4.3.</span> <span class="nav-text">修饰类对象</span></a></li></ol></li></ol></div>
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